1. FIELD OF THE INVENTION
This invention relates to a process for removing mercury from a cyanide liquor of a precious metal, cyanide leach solution. In particular this invention relates to a process for removing mercury from a secondary circuit cyanide liquor by reacting the mercury with a sulfide ion-providing compound and flocculating the mercury sulfide contained in the secondary circuit cyanide liquor.
2. DESCRIPTION OF THE PRIOR ART
Cyanide leach, carbon-in-pulp systems commonly used in precious metal extracting plants combine cyanide leaching, carbon adsorption, and carbon desorption steps to recover precious metal from refractory ores. The steps in precious metal extraction processes involving the initial cyanide leaching constitute the "primary circuit" of the processes. The steps in precious metal extraction processes involving the desorption of the precious metal from activated carbon constitute the "secondary circuit" of the processes. The secondary circuit also employs a cyanide-containing solution.
U.S. Pat. Nos. 4,188,208 to Guay and 4,289,532 to Matson et al. disclose "binary" or two circuit systems for recovering precious metals from carbonaceous ores. Carbonaceous ores contain refractory carbon material that inhibits or substantially reduces the effectiveness of conventional cyanidation techniques for extracting the precious metals from the ore.
The Matson patent, which is herein incorporated by reference, is illustrative of a cyanide leach, carbon-in-pulp precious metal recovery system. In the Matson patent the primary circuit involves the cyanide leaching of precious metal values from precious metal-containing, carbonaceous ores and the adsorption of those leached precious metal values onto activated carbon. The dissolved cyanide content of a primary circuit is considered to be low and is typically in a concentration between about 0.01 percent and about 0.1 percent by weight of the ore. A secondary circuit as typically used with the process disclosed in the Matson patent involves the desorption of the adsorbed precious metals from the precious metal-loaded activated carbon by a cyanide stripping solution. The secondary circuit also includes the recovery of the desorbed precious metals from the desorption solution. The dissolved cyanide content of the secondary circuit desorption liquor is significantly higher than the dissolved cyanide content of the primary circuit. The dissolved cyanide content of the secondary circuit is typically between about 0.5 percent and about 2.0 percent by weight of the ore. A high cyanide concentration increases the solubility of precious and other metals.
Mercury is often found in gold-containing ores and its presence results in problems throughout gold recovery plants which use cyanide leaching and carbon adsorption. Both mercury and gold are readily solubilized when leached with a cyanide solution. Solubilized mercury and gold are both adsorbed onto activated carbon when the activated carbon is contacted with a mercury and gold-containing, cyanide solution. The adsorption of mercury onto the activated carbon reduces the availability of the activated carbon for the adsorption of the precious metal. Subsequent desorption of the metal-loaded activated carbon by a cyanide stripping solution results in the desorption of both mercury and gold. Mercury therefore becomes a contaminant of the precious metal.
One conventional process for the removal of mercury from mercury-contaminated gold includes a distillation operation. In the distillation operation the gold and mercury recovered from a conventional cyanide leach, carbon-in-pulp system are heated in a furnace to a temperature that is sufficient to vaporize or "distill off" the mercury from the gold. The distilled mercury is collected in a condenser and can be sold or otherwise disposed in an environmentally safe manner. This distillation operation is expensive due to the high cost of equipment and energy. The distillation operation produces toxic mercury vapors. For these reasons the removal of as much mercury as possible from a cyanide leach system prior to a distillation operation is desirable to reduce the magnitude of a mercury distillation operation.
U.S. Pat. No. 4,256,707 to Flynn et al. discloses a process for selectively removing mercury from gold-cyanide solutions. A mercuric sulfide precipitate is formed by adding silver sulfide (Ag.sub.2 S) , zinc sulfide (ZnS), or iron sulfide (FeS) to the gold-cyanide solution. The examples in Flynn disclose cyanide solutions containing 0.05% cyanide by weight. This cyanide concentration is representative of cyanide concentrations in the primary circuits of cyanide leach, gold recovery systems.
There are several disadvantages associated with the precipitation of mercuric sulfide from primary circuit cyanide solutions such as those disclosed in the Flynn patent. First, when sulfides are added to the dilute cyanide solution of the primary circuit, a diffuse, hard-to-settle mercuric sulfide precipitate is produced which Flynn states "flocculated after about 40 hours". A solution existing as a colloidal suspension for 40 hours is representative of a "nondeflocculated system". This is evidenced by the fact that only 60 percent of the mercury in the original solution in the Flynn patent was removed following centrifugation and filtration when Na.sub.2 S was used as a sulfiding agent. Separating a diffuse mercuric sulfide precipitate from solution by centrifugation is not a process readily adapted to a commercial scale gold extraction process. A second disadvantage of sulfide precipitation of mercury in the primary circuit of a gold extraction process is the loss of gold from the dilute cyanide solution. The low concentration of cyanide in a primary circuit results in decreased solubility of the gold-cyanide complexes when compared to the strongly solubilized gold-cyanide complexes of a secondary circuit. A larger portion of the complexed gold in a primary circuit precipitates upon the addition of sulfide to the liquor than occurs upon the addition of sulfide to a secondary circuit.
The industry lacks an economical and efficient process for removing mercury from a secondary circuit of a cyanide leach, carbon-in-pulp precious metal recovery system. The industry also lacks a process for the removal of mercury from a cyanide solution that minimizes problems associated with subsequent gold purification steps such as distillation.